Controlling electric machines, such as motors and generators, in fault conditions is important to providing a safe and controllable machine. Some types of machines, by reason of their architecture, are inherently more difficult to control. For example, a permanent magnet alternator which is continuously driven while suffering an internal short-circuit in its windings can present a particular concern, especially in high power applications. Consequently, the prior art presents many solutions to the control of electric machines in short-circuit situations.
The prior art shows both electrical/electronic and mechanical means for responding to or preventing electric machine internal faults. One example of a mechanical solution in presented in U.S. Pat. No. 4,641,080 to Glennon et al., in which the stator of a permanent magnet generator is divided into two semi circular halves which, in the event of a detected fault, are opened by an actuator motor to increase the air gap between the stator and the rotor. Increasing the gap reduces the magnetic flux in the stator and thereby prevents burn out in the windings. The mechanisms involved, however are complex, expensive and present reliability issues to the designer. An improved solution is therefore desired, and it is an object of this invention to provide improved solutions to the problem of electric machine control.